Many electronic systems, such as telecommunications or computing systems, use differential signaling to transmit information electronically. Typically, differential signaling enables information to be transmitted over two complementary signals sent over two separate wires. When a receiving device receives a differential signal, the receiver may decode the signal by comparing the two signals to measure the difference. Differential amplifiers are often used in differential signaling to boost differential signals before transmission.
In boosting the differential signals, differential amplifiers may increase the output range and bandwidth of a signal path. However, some existing differential amplifier designs only provide limited improved output ranges because of other factors, such as common mode stability issues. For example, FIG. 1 shows an existing dedicated differential-in differential-out amplifier. In this example, the input FET transistors M1 to M4 are cross-coupled so that each pair of inputs drive the outputs differentially, or out-of-phase. During this cross-coupling, the common mode input is defined as taking both the positive inputs of a differential amplifier and moving them in phase with each other through transistors M1 and M4, while moving them out of phase with the inverting inputs M2 and M3. Since the drain currents of transistors M1 and M3 are opposite and equal and the drain currents of transistors M2 and M4 are also opposite and equal, there is no net current out of the input stage, which explains the need for a common mode feedback circuit with transistors Mc1-Mc7.
A second existing design is shown in FIG. 2. In FIG. 2, two op-amps 201 and 202 are connected as a differential-in differential-out amplifier. In this circuit, the input common mode voltage VCM flows through the circuit to the output, resulting in a common mode voltage gain of 1. The differential gain is set by the resistor ratio, G=(1+Rf/Rg). However, in certain instances, such as driving a subscriber line or power line network using CMOS digital-to-analog converters operating on low voltage (1.8V-3.3V) supply rails, gains of 5 to 10, or more are needed. Although decompensating the amplifiers 201 and 202 may result in higher close loop bandwidth and lower distortion in these instances, unity gain stability is required in the amplifiers 201 and 202 of this example, preventing the amplifiers 201 and 202 from being decompensated.
A third existing design is based on cross-coupling compensation capacitors to get the benefits of higher closed loop bandwidth and lower distortion. In such a design, the unity gain crossover frequency may be proportional to the product of the inverse input transconductance, 1/GM, of amplifier 302 and the compensation capacitance, Ccomp, where the crossover frequency fc=½*π*(1/GM)*Ccomp. When a typical op-amp is connected in a closed loop gain of G, it may remain stable as its compensation capacitance is reduced to (1/G)*Ccomp, thereby improving bandwidth, slew-rate, and distortion performance. However, the need to maintain common mode voltage stability requires a higher value of Ccomp, which prevents attainment of these performance improvements. Cross-coupling compensation capacitors circumvents this limitation by providing different compensation capacitances for the common mode and differential signals and therefore two different cross over frequencies. Although providing a larger capacitance for the common mode may preserve unity gain stability, and the smaller differential capacitance may provide increased bandwidth, in some instances it may be desirable to provide broader bandwidth for differential operation while providing lower bandwidth and stability for the common mode signal without manipulating capacitances.
Thus, there is a need for additional devices and methods for enhancing the dynamic range of differential signals while maintaining the stability of common mode signals using dual op-amps without manipulating capacitance.